This invention is directed to commercial or residential heat pump systems that provide heating or cooling of a comfort zone, as required, and which can also provide water heating. The invention is more particularly directed towards an improved electrical power management system in which the power draw from individual heat pump systems can be reduced gracefully during times of peak electrical demand, in response to a utility demand limit signal furnished from the electric power utility to its electrical customers.
Integrated heat pumps are often employed to provide heating or cooling, as needed, to a residential or commercial comfort zone, i.e., the interior of a residence, office complex, hospital, or the like. Integrated heat pumps can also be employed to heat water. A heat pump system for air conditioning, comfort zone heating, and water heating is described in U.S. Pat. No. 4,766,734. Systems of this type can have a number of modes of operation, such as air conditioning alone, space heating alone, water heating alone, air conditioning with water heating, and comfort zone heating with water heating. Additional modes, such as a defrost cycle can also be employed. For comfort zone heating and water heating, resistive elements are employed as auxiliary heating elements for use at times when the heat pump alone cannot produce sufficient heating of the comfort zone or produce enough hot water in the water heater.
During times of regional temperature extremes, especially during warm summer months, the electrical demand for a given power utility may approach the maximum electrical capacity of the utility. If demand is left unchecked, the total power draw for a community could exceed the capacity of the utility to provide electrical power. During times of peak customer loading, the utility must take steps to reduce customer demand, otherwise power blackouts or brownouts can occur, which can be accompanied by catastrophic loss of power to regions of the electrical utility's service area.
One approach that has been proposed to address this problem is for the utility to supply a demand limit signal to its customers whenever the power demand exceeds threshold value. When the utility supplies the demand limit signal, the customer's air conditioning equipment is cycled off and on periodically and at staggered intervals to reduce the total current draw from the customer base. That is, for the customers that receive the demand limit signal, the air conditioning compressor motors, for example, may be turned off automatically, and then automatically restarted at some later time. This reduces the number of compressors operating at any one time, and thus reduces the total current draw that has to be supplied by the electrical utility.
This system has some merit from the utilities' standpoint, but has a number of drawbacks from the customers' standpoint. The residence or commercial building air conditioning system, being simply turned either off or on depending on the state of the demand limit signal, can produce wide swings in temperature from the off interval to the on interval. Also, the homeowner or building manager can tend to "anticipate" a peak-period demand limit, and reduce the temperature in the building prior to the time of intermittent operation. This produces an even greater load than normal in times when a power shortage may be possible. Also, the reduced temperature set point will cause a greater electrical load (with accompanying large temperature swings) during times when the utility does institute the demand limit.
Thus, a system has been sought which would produce a more graceful reduction in air conditioning performance at times of peak electrical power loading, and some means have been desired to prevent customers from defeating the demand limit by simply reducing the thermostat set point.